1. Field of the Invention
The present invention relates to a levitation (electromagnetic) melting crucible. More particularly, the present invention relates to a crucible to be employed for levitation melting, in which a metallic material introduced to the crucible is melted in no contact with the inner wall surface of the crucible to give a contaminant-free molten metal.
2. Description of the Related Art
There is known the levitation melting method as a melting method which can prevent, when a metallic material of various kinds introduced to a melting crucible is to be melted therein, the resulting molten metal from being contaminated due to chemical reactions occurring when it is brought into contact with the inner wall surface of the crucible and which can thus achieve improvement in the quality of molten metal. In this levitation melting method, the molten metal formed in the crucible is levitated by an electromagnetic force to be in no contact with the inner wall surface of the crucible so as to prevent migration of contaminants from the crucible into the molten metal.
FIG. 8 shows a crucible employable in the levitation melting method. The crucible 10 has a cylindrical main body 10 with a closed bottom, the circumferential wall of which contains a plurality of vertical slits 14 defined at predetermined intervals in the circumferential direction to divide the circumferential wall into several segments 16. These segments 16 are designed to be electrically insulated from one another by an insulating material 18 such as a refractory ceramic filled in the slits 14. Induction coils 20 are disposed to surround the crucible 10, and when a high-frequency current is applied to these coils 20, a material 22 introduced into the crucible 10 is heated and melted, and also the thus obtained molten metal 22a is designed to be levitated to be in no contact with the inner wall surface of the crucible 10 by the electromagnetic force penetrating into the crucible. Incidentally the reference number 11 shown in FIG. 8 denotes a solidified portion of the material 22.
In the levitation melting crucible 10 described above, it is advantageous to secure a large opening width for the slits 14 in order to facilitate penetration of magnetism into the crucible 10 and to ensure levitation of the molten metal 22a in no contact with the inner wall surface of the crucible 10. However, if a large opening width is secured for the slits 14, the area of the insulating material 18 packed in the slits to be exposed to the inside of the crucible is enlarged. Accordingly, when an active metal having a high melting point such as titanium is to be melted, the molten active metal is readily brought into contact with the insulating material 18, leading to increased liability to contamination to be caused by chemical reactions with the insulating material 18. More specifically, in the standpoint of preventing contamination of the molten metal, it is advantageous to secure a small opening width for the slits 14. It should be noted here that, when a small opening width is secured for the slits 14, the operation of packing the insulating material 18 into the slits 14 becomes difficult to lower working efficiency, disadvantageously.
As described above, in order to fully achieve both improvement in penetration of magnetism into the crucible and prevention of contamination of the molten metal, both of these contradictory requirements described above must be satisfied. However, the prior art crucible cannot be said to have overcome both of these requirements but one of them is laid aside under the present circumstances.